In this article we will discuss about Cell Surface Antigens:- 1. Meaning of Cell Surface Antigens 2. Types of Surface Antigens 3. Functions.
Meaning of Cell Surface Antigens:
Cells are antigenic. This means that when cells of one species are injected into another species, the recipient will first identify the injected cells as being of foreign origin and start producing antibodies to interact specifically with the alien cells. Therefore, the foreign cell that provokes antibody production in the recipient is called antigen.
If whole cells are injected, many cell surface components—like protein, carbohydrates or some combination of the two— may act as cellular antigen of the foreign cell. Several cell surface antigens have been studied in detail—the ABO blood group antigens, the MN blood group antigens, histocompatibility antigens etc. The most commonly studied cell surface antigens are discussed below.
Types of Surface Antigens:
(i) The ABO Blood Group Antigens:
The recognition of any blood grouping or specific type within it depends on the ability to detect the presence or absence of specific antigens on the red blood cells. Such antigens are found in the membrane of red cells or erythrocytes. In the human, classification of a person as blood type A, B, etc. is possible because of the presence of the detectable antigens.
If a certain antigen is present in the red blood cells, these may be clumped by the corresponding antibody when it is present. If the specific antigen is not present in the red blood cells, the corresponding antibody anti-A or anti-B is present in the blood serum.
A person of type AB blood is born with both A and B antigens in the red blood cells but no anti-A or anti-B antibody are found in the serum. The type O person lacks both A and B antigens into the membrane of the red blood cells, but the serum contains both antibodies anti-A and anti-B.
All these antigens are under genetic control so that an individual may possess either A or B antigen, or both, or neither. When neither antigen is present, the individual is called O type. However, individuals with O type erythrocytes possess an antigen called H which is the structural foundation on which A and B antigens are built.
The antigens of ABO system are glycolipids, with the oligosaccharide portion of the glycolipid responsible for antigenic properties. The oligosaccharide antigen is covalently attached to sphingolipids which are immersed in the bi-molecular lipid leaflet of the plasma membrane. There are two types of oligosaccharide core chains which, ultimately, give rise to two subgroups of each antigenic type.
The difference is due to the type of linkage between galactose and N-acetylgalactosamine at the core of the oligosaccharide. In type I chain, galactose is attached with either N-acetyl glucosamine or N- acetylgalactosamine by β-1, 3 linkage, whereas in type II chain galactose is attached with N- acetyl glucosamine by β-1, 4 linkage.
Erythrocytes that are O type, does not contain galactose or N-acetyl glucosamine or N- acetylgalactosamine as side chain and fucose is linked a-1, 2 galactose, a structure that constitutes the H antigen. From this unit the A and B antigens are enzymatically generated by glycosyltransferases that attach either on N- acetylgalactosamine unit (type A) or a galactose unit (type B).
The level of genetic control is exercised by presence or absence of glycosyl transferees that modify the H antigen; adding either an A or B determinant.
(ii) The MN Blood Group Antigens:
The second major blood group system in human is MN system. In this case, the oligosaccharide is linked to a protein, glycophorin. It has two units. One “type of unit (A) is attached through asparagine, and the other unit (B) through serine or threonine. Variations on these structures exist.
They characteristically contain terminal sialic acid (N-acetylneuraminic acid). Both M and N determinants are destroyed when erythrocytes are treated with neuraminidase, an enzyme that removes sialic acid.
(iii) Histocompatibility Antigens:
Histocompatibility antigens are tissue cell surface proteins that differ from individual to individual. They are recognised by the mechanism of tissue graft rejection. There are different number of antigenic combinations possible on tissue surfaces and this is the basis for the difficulties faced in getting a good tissue match for skin or organ transplantation surgery.
Some of the histocompatibility antigens are strong and others are weak. H-2 antigens in mice and HLA-antigen in humans have been studied in detail. Both these systems have many genetic variants. The H-2 antigen in mice is strongly antigenic. The intact H-2 antigen is made of two chains of which one is heavy or long and other is light or short.
The light claim is actually a protein and it is called β-2-micro-globulin. The heavy and light chains interact but not by covalent bonds. The proteolytic enzyme papain cleaves the heavy chain in such a way that a water-soluble portion called the Fs fragment, is released and a small piece, the Fm fragment is left in the membrane.
Partial sequences have been worked out for the heavy chains of both H-2 and HLA antigens and similarities are seen between the two types of heavy chains.
Functions of Surface Antigens:
The chemical nature of several types of surface antigen are now known. Surface antigens have some biological importance and are involved in many functions. For examples, H-2 antigens may function during the mechanism of immunological surveillance.
If any change or modification in cell surface structures takes place—either by mutation or other means— the surface structures become abnormal. The abnormalities arising in cell surface are readily recognised by wandering lymphocytes in the immune system.
When a defective cell is found by the lymphocytes, a message is sent to the lymph modes where a class of lymphocytes called killer T cells are activated and, ultimately, destroy the abnormal target cell. The target cell must have an H-2 antigen plus an abnormal or modified antigen.
The normal (H-2) and the abnormal antigens may be present independently, or they may form a hybrid surface structure. In any case they are then recognised or they may attract the lymphocytes without being physically attached to one another. In any event, in the absence of H-2 antigen, an abnormal cell will avoid destruction and proliferate.